Recently, there has been commercially available an ultrasonic diagnostic apparatus which acquires echo signals from an object by three-dimensionally scanning the object. This ultrasonic diagnostic apparatus can generate and display a three-dimensional image (for example, a rendering image) by generating three-dimensional data based on echo signals. To generate a three-dimensional image, this apparatus executes, for example, rendering processing (for example, volume rendering or surface rendering) for three-dimensional data. A viewpoint, line of sight, visual angle, projection region, and the like used for rendering processing are set for three-dimensional data. After the above setting, the apparatus generates a rendering image (for example, a parallel projection image or a perspective projection image) by executing parallel projection or perspective projection.
As one application example of a rendering image display method, there is available a display method of continuously moving the viewpoint set in an organ and consecutively displaying rendering images corresponding to the moved viewpoint. In this display method, for example, when handling three-dimensional data concerning an organ having a luminal form (to be referred to as a luminal organ hereinafter), a viewpoint is set in the lumen of the luminal organ. A line of sight is set in the extending direction of the lumen with the set viewpoint being a start point. In addition, a predetermined visual angle centered on the set viewpoint is set. The apparatus then executes perspective projection by using the set viewpoint, line of sight, and visual angle. The viewpoint is moved along the lumen in accordance with operation by the operator or at a predetermined velocity. Moving this viewpoint makes it possible to display an image of the interior of the lumen. This display method allows the operator to thoroughly observe the inner wall of the lumen.
As shown in, for example, FIG. 13, however, when the apparatus generates three-dimensional data after moving an ultrasonic probe, the viewpoint in this display method is located at a position fixed on a coordinate system concerning the three-dimensional data regardless of the position of the ultrasonic probe. This raises a problem that when a viewpoint is moved accompanying the movement of the ultrasonic probe, the position of the moved viewpoint may fall outside the interior or the lumen. When this happens, no perspective projection image of the interior of the lumen to be observed is displayed. In addition, assume that the apparatus has generated three-dimensional data by using a mechanical four-dimensional probe designed to execute three-dimensional scanning by swinging a one-dimensional array in a direction perpendicular to the array direction of a plurality of transducers. In this case, when the apparatus executes this display method, the frame rate is insufficient in continuous, real-time display of rendering images.